Julia: Printing vectors of NamedTuples as a markdown table

[bramtayl]

A vector of NamedTuples has the potential to become a "lightweight table". See https://github.com/JuliaLang/julia/issues/12131#issuecomment-325281471. To that end, printing a vector of NamedTuples with the same fields as a markdown table is a good start. This is a convenient way to view the contents of a vector of NamedTuples regardless. In Keys.jl I had the beginning of the code for this. I was waiting for a windows binary to come out with NamedTuples in order to be able to port the code, but I got bored. So anyway here's the code I had below. I'm somewhat indifferent about the actual implementation details.

```{r}
export KeyedTable
const KeyedTable = AbstractVector{T} where T <: KeyedTuple{K, V} where V <: Tuple where K <: Tuple

show_row(io, atuple) = begin
print(io, "| ")
join(io, atuple, " | ")
println(io, " |")
end

show_row(io, atuple, widths, n) =
show_row(io, map(rpad, atuple, widths)[1:n])

function Base.summary(t::KeyedTable)
"$(length(t)) x $(length(type_keys(eltype(t)))) keyed table"
end

struct Repeated
text::String
number::Int
end

function Base.show(io::IO, r::Repeated)
text = r.text
for i in 1:r.number
print(io, text)
end
end

function Base.showarray(io::IO, t::KeyedTable, ::Bool)
println(io, summary(t))
if length(t) == 0
nothing
else
row_number, column_number = displaysize(io)
limit = get(io, :limit, false)
names = string.(type_keys(eltype(t)))
# subset rows for long arrays
subset =
if limit
t[1:min(row_number - 3, length(t))]
else
t
end
rows = map(subset) do row
string.(row.values)
end
# find maximum widths for rows
row_widths = mapreduce(
row -> map(strwidth, row),
(x, y) -> map(max, x, y),
rows
)
# and then also for names
widths = map(max, map(strwidth, names), row_widths)
# figure out how many columns we can safely print
if limit
n = findfirst(x -> x > column_number - 2, cumsum([(widths .+ 3)...])) - 1
if n == -1
n = length(widths)
end
else
n = length(widths)
end

    if n > 0
        show_row(io, names, widths, n)
        show_row(io, (Repeated("-", i) for i in [widths...][1:n]))
        for row in rows
            show_row(io, row, widths, n)
        end
        nothing
    else
        nothing
    end
end

end
```

[JeffBezanson]

I think this is a good idea. The usual printing of arrays of named tuples is pretty annoying, repeating the names on every line. This also allows implementing things like #12131. What's your concern, @ararslan ?

One issue is that we don't want too many different ways of printing tables. We should perhaps move DataFrames' printing code into Base as a showtable function.

[ararslan]

My concern is that it's too specific; arrays of named tuples could potentially be used for things other than table-like structures. Plus, what do we do if a given element in the array has different names than the others? That would mess up the table formatting. And it would be weirdly inconsistent to print things nicely only in the case of an array where each element has the same set of names.

[bramtayl]

I think you should be able to tell whether or not the names are consistent from the type signature alone.

[ararslan]

Yes, but that doesn't change or address anything I said above.

[JeffBezanson]

weirdly inconsistent to print things nicely only in the case of an array where each element has the same set of names

I don't see it as inconsistent, since having the same names is what makes the nicer printing make sense. Similar to how we will print the type of each element if the array is heterogeneous, but not if all elements have the same type.

[ararslan]

Similar to how we will print the type of each element if the array is heterogeneous, but not if all elements have the same type.

?

julia> Any[1, 1.0, "1"]
3-element Array{Any,1}:
 1
 1.0
  "1"

[JeffBezanson]

Well, that's how it should work; see #24651. It doesn't apply to all types since many types like strings don't ever have a type as part of their representation.

[rfourquet]

I'm not sure yet, but I think I prefer a dedicated showtable function or KeyedTable struct which is not an alias to NamedTuple. My 2 small concerns is how the formatting will be degraded when one line doesn't fit on one screen line, and the big difference between the printing of two NamedTuple arrays depending on whether all the elements have the same names. Also FWIW, with the PR linked by Jeff, it's already easy to have not the field names repeated if they all have the same.

[bramtayl]

How about a table struct wrapper for a vector named tuples which prints as a markdown table?

[bramtayl]

An updated version now that the windows 0.7 binaries are out:

const Table{T} = AbstractVector{NamedTuple{T, T1}} where {T, T1 <: Tuple}

Base.names(t::Table{T}) where T = T

function Base.summary(t::Table)
    "$(length(t)) x $(length(names(t))) Table"
end

function Base.showarray(io::IO, t::Table, ::Bool)
    println(io, summary(t))
    if length(t) != 0
        display_length, display_width = displaysize(io)
        limit = get(io, :limit, false)
        table_names = string.(names(t))
        number_of_columns = length(table_names)
        rows = map(
            if limit
                t[1:min(display_length - 3, length(t))]
            else
                t
            end
        ) do row
            string.(Tuple(row))
        end
        unshift!(rows, table_names)
        if limit
            n = findfirst(
                x -> x > display_width - 2, 
                cumsum([(mapreduce(
                    row -> map(textwidth, row),
                    (x, y) -> map(max, x, y),
                    rows
                ) .+ 3)...])
            ) - 1
            if n != -1
                number_of_columns = n
            end
        end
        if number_of_columns > 0
            Base.show(io, Markdown.MD([Markdown.Table(
                map(rows) do row
                    [row...][1:number_of_columns]
                end, 
                repeat([:r], inner = number_of_columns)
            )]))
        end
    end
end

[bramtayl]

Still think this might be nice, but I guess a little unexpected